In conventional methods for the production of wet process phosphoric acid, finely ground phosphate rock is reacted with dilute phosphoric acid (10% P.sub.2 O.sub.5 to 25% P.sub.2 O.sub.5) and sulphuric acid which may or may not be diluted. The reaction product is leached (i.e., digested or reacted with the acid) to yield a crude aqueous phosphoric acid solution in which is suspended a substantial quantity of solid impurities. This slurry is filtered to separate most of the undissolved gypsum and other solid impurities to yield a crude (22% P.sub.2 O.sub.5 to 35% P.sub.2 O.sub.5) aqueous product sometimes known as the number one filtrate. This filtrate contains suspended, finely divided, impurities that were not removed during the solid-liquid separation step plus solids that have crystallized after filtration. The solution also contains dissolved impurities. Reference is made to Chapter 16 of Volume II, Phosphorus and its Compounds, Van Wazer, Interscience Publishers (1961).
The problems occasioned by suspended and dissolved impurities are particularly acute with respect to wet process phosphoric acid. Variable quantities of impurities in the phosphate rock such as organics, calcium, potassium, sodium, aluminum, iron, strontium, titanium, silicon, uranium, vanadium, fluorine, magnesium, etc. are put into solution during the reaction of the phosphate rock with sulphuric and/or phosphoric acid. Standing, cooling, or concentration of the phosphoric acid solution results in additional solids being formed and precipitated to form a sludge consisting mainly of complexed iron, aluminum, potassium, sodium, calcium, etc.; and phosphates, fluorides, silicates, sulphates, etc. This is known in the art as post-precipitation.
Concentrated wet process phosphoric acid, upon standing or during shipment in tank cars or the like, deposits a layer of solid matter forming sludge as above described, which renders handling stored or shipped acid exceedingly difficult and frequently economically unfeasible. Some solids may be removed from the crude solution by conventional filtration or centrifugation. With respect to conventional filtration, however, the concentrated acid is very viscous and difficult to filter. Further, the solids are of such nature that they readily blind the filter cloth and the solids are difficult to remove from the filter. With respect to conventional centrifugation, many of the finely divided solids will not be removed, particularly the organic particles, which are lighter and are not removed with the larger heavier inorganic solids from the solution.